1. Field
This application relates to technology for data storage.
2. Description of the Related Art
A variety of materials show reversible resistivity-switching behavior, and as such may be suitable as use for memory elements. One type of material having reversible resistivity-switching behavior is referred to as resistance change memory (ReRAM). Transition metal oxides have been proposed for ReRAM. Upon application of sufficient voltage, current, or other stimulus, the reversible resistivity-switching material switches to a stable low-resistance state, which is sometimes referred to as SETTING the device. This resistivity-switching is reversible such that subsequent application of an appropriate voltage, current, or other stimulus can serve to return the reversible resistivity-switching material to a stable high-resistance state, which is sometimes referred to as RESETTING the device. This conversion can be repeated many times. The low resistance state is sometimes referred to as an “on” state. The high resistance state is sometimes referred to as an “off” state. For some switching materials, the initial state is low-resistance rather than high-resistance.
These switching materials are of interest for use in nonvolatile memory arrays. One type of memory array is referred to as a cross-point array, which is a matrix of memory elements typically arranged along x-axes (e.g., word lines) and along y-axes (e.g., bit lines). A digital value may be stored as a memory resistance (high or low). The memory state of a memory cell can be read by supplying appropriate voltages to the bit line and word line connected to the selected memory element. The resistance or memory state can be read as an output voltage of the bit line connected to the selected memory cell. One resistance state may correspond to a data “0,” for example, while the other resistance state corresponds to a data “1.” Some switching materials may have more than two stable resistance states.
Non-volatile memories formed from reversible resistivity-switching elements are known. For example, U.S. Patent Application Publication 2006/0250836, filed May 9, 2005 and titled “REWRITEABLE MEMORY CELL COMPRISING A DIODE AND A RESISTIVITY-SWITCHING MATERIAL,” which is hereby incorporated by reference herein in its entirety, describes a rewriteable non-volatile memory cell that includes a diode coupled in series with a reversible resistivity-switching material such as a metal oxide or metal nitride. The diode serves as a “steering element” to control which memory cells are programmed (e.g., SET or RESET) and read.
Two proposed modes of switching the memory cells between SET and RESET are unipolar and bipolar switching. In bipolar switching, the low resistance state is established by applying a voltage having one polarity and the high resistance state is established by applying a voltage having the opposite polarity. In unipolar switching, switching between the low resistance state and high resistance state is accomplished by applying voltages of the same polarity, although perhaps different magnitudes. For example, unipolar switching may depend on the amplitude of the applied voltage, but not the polarity. Note that with bipolar switching both the polarity and amplitude of the voltage may be different.
One theory that is used to explain the switching mechanism is that one or more conductive filaments are formed by the application of a voltage to the memory cell. The conductive filaments lower the resistance of the memory cell. Application of another voltage may rupture the conductive filaments, thereby increasing the resistance of the memory cell. Application of another still another voltage may repair the rupture in the conductive filaments, thereby decreasing the resistance of the memory cell once again.
The reversible resistivity-switching element may be in the high resistance state when it is first manufactured. The term “forming” is sometimes used to describe putting the reversible resistivity-switching element into a lower resistance state for the first time. Thus, the initial formation of the conductive filaments is sometimes referred to as “forming.” The rupture of the filaments is sometimes referred to as RESETTING. The repair of the rupture of the filaments is sometimes referred to as SETTING. The formation of conductive filaments may be associated with either unipolar or bipolar switching. However, one theory explains unipolar switching as resulting from conductive filaments.
Another theory to explain switching behavior is that application of voltages alters the contact resistance at an interface between the reversible resistivity-switching element and an electrode. For example, application of a voltage of one polarity may decrease the contact resistance, whereas application of a voltage of the opposite polarity may increase the contact resistance. One theory to explain the change in contact resistance is electrochemical migration of oxygen vacancies to the interface. Another theory to explain the change in contact resistance is trapping of charge carriers at the interface (electrons or holes). It has been proposed that bipolar switching may occur, at least in part, due to changes in contact resistance.
As previously mentioned, the term “forming” is sometimes used to describe putting the reversible resistivity-switching element into a lower resistance state for the first time. Thus, the initial reduction of contact resistance may be referred to as “forming.” Application of another signal which increases the contact resistance may be referred to as RESETTING. Application of still another signal which decreases the contact resistance again may be referred to as SETTING. Thus, note that both forming and SETTING refer to lowering the resistance of the reversible resistivity-switching element.
Note that switching behavior might be explained by other theories than those above. Thus, any of the reversible resistivity-switching elements described herein are not limited to the theories for switching behavior described herein. Also note that forming is generally used to describe the first time that the resistance is altered to a lower resistance state. However, forming is not necessarily limited to such an interpretation.